This application is directed to early detection of skin abnormalities, in particular pressure ulcers. The phrase “early detection” means detecting a pressure ulcer before the pressure ulcer is visually discernible and/or identifiable as a pressure ulcer by visual inspection by a person unaided by anything other than commonplace corrective lenses. Early detection also includes detecting precursors to pressure ulcers. A precursor to a pressure ulcer is a condition that does not qualify as a pressure ulcer but whose presence indicates that a pressure ulcer is likely to develop. Accordingly, the methods and apparatuses described herein for detecting pressure ulcers may also be referred to as methods and apparatuses for predicting pressure ulcers or for detecting precursors to pressure ulcers.